The invention relates to a method for the manufacture of double-sided metallized ceramic substrates according to a direct-bonding process. Double-sided metallized ceramic substrates are generally known. Such metal-ceramic substrates are particularly useful for power semiconductor modules as circuit carriers. These circuit carriers include a top and underside with a ceramic provided with a Cu-metallization, with at least one metallized side having a circuitry structure. The known method for the manufacture of these metal-ceramic composites, by means of eutectic bonding, is generally understood as direct-bonding process or Direct Copper Bonding (“DCB”).
U.S. Pat. No. 3,994,430 and EP 0 085 914 describe a method for the manufacture of metal-ceramic substrates according to a direct-bonding process. The known manufacturing processes have in common that the formation of connections between the metal plate or foil and the ceramic is effected by means of a liquid-phase bond process. For this purpose, use is made of local melting of an intermediate layer applied onto the metal plate, of which the melting point (eutectic) lies below the melting point of the metal and the ceramic.
The direct-bonding process described in these references includes the following steps:                Application of a suitable layer onto at least one side of an unstructured metal plate to create a eutectic melt-on layer        Application of the unstructured metal plate with the melt-on layer onto the upper side of an unstructured ceramic        Heating of metal plate and ceramic to a temperature which lies above the melting point of the melt-on layer and below the melting point of the materials which are to be bonded        Cooling of the composite structure to room temperature, and        if appropriate, grinding and polishing of the bonded metal layer to eliminate local uneven areas resulting from the bonding process.        
With the method of U.S. Pat. No. 3,994,430, there is the disadvantage that the bonding of the metal plates to the top and underside of the ceramic because with simultaneous bonding of both metal plates, the lower metal plate tends to adhere to the substrate carrier, and the subsequent detachment leads to damage of the metal-ceramic substrate. Accordingly, at least two process steps are required for the manufacture of metal-ceramic substrates metallized on both sides. This is time-consuming and costly.
DE 102004 056879 describes a method for the simultaneous bonding of two metal foils to a ceramic. With this method, the intention is that the adhering of the lower metal foil to the substrate carrier is avoided by the interspacing of an additional separation layer, typically a porous material. The disadvantage is that the separation layer required must first be applied with suitable means, and after the bonding process, must be removed from the metal surface with a suitable process. This approach also involves additional time and cost.
A further problem with the joining of metal-ceramic substrates by means of a liquid-phase bonding process lies in the fact that the bonding between the metal plate and the ceramic substrate does not take place across the entire surface. The inclusions which occur in the boundary surface between the metal and ceramic exert a negative effect on the partial discharge resistance of the substrates, causing local interference with the thermal and mechanical bond between the metal and the ceramic.
U.S. Pat. No. 4,409,278, U.S. Pat. No. 4,860,939, and DE 102004 033933 describe methods with which the bubble formation is intended to be reduced, or the bonded surface is intended to be enlarged. U.S. Pat. No. 4,409,278 proposes that “venting lines” be located in the metal plate or ceramic. The disadvantage of this approach is that, during the bonding processes required for the building up of the power semiconductor modules, process fluids can collect in these structures, such as flux materials during soldering. Process fluids may emerge during the use of the modules, and reduce the service life of the components. In addition to this, the “venting lines” reduce the thermal and electrical link between the metal and ceramics.
U.S. Pat. No. 4,860,939 proposes that bubble formation be avoided by intensive cleaning of the surfaces of the metal plate and ceramic. The intention is that this should be attained, in particular, by the in-situ formation of the reaction layer necessary for the liquid phase bonding. While appealing in theory, in practice, however, there are other causes of bubble formation apart adherence of dirt contamination.
With the method described in DE 102004 033933, the intention is that the bubbles incurred during the bond process should, in a subsequent process, be pressed out of the boundary surface by very high pressure (400 to 2000 bar) and high temperature (450 to 1050° C.). A disadvantage is that this process for removing the bubbles is time consuming and cost-intensive. As a result the method is largely unsuitable for mass production.